It could be something fungal/bacterial or viral in one's body. It could also be because of accumulation of heavy metals. I have had testing in all areas and found out that I showed up negative to any kind of living parasite and so on. I did a heavy metal test and found out that I have extreme lead and aluminum poisoning and elevated levels of mercury, nickel and cadmium. I have had problems with dark circles since the time my family moved into a very old house. I was 14 and now I am 28. I was more than likely exposed to the lead in the old house and the aluminum from eating out all the time in restaurants that use aluminum cookware. Stainless steel is not always just that. Most stainless steel cookware has an aluminum core that can be brought to surface after several uses of it. Heavy metals are soooooooooo horrible for the body and there is an abundance of them everywhere around us besides the pcb's which are the worse!!! Heavy metals wreak havoc with every organ in ones body including the kidneys. That would explain the dark circles. In Eastern medicine the sites under and in the corner of the eyes are signs of kidney or pancreas problems. Heavy metals also cause blood sugar, hormonal and circulatory problems. Left untreated you can have all sorts of problems like metabolic syndrome, diabetes, and cancer!!! It's amazing how our bodies speak to us and give us signals on the outside if we look. Dark circles are bad and a sign. I wish that I would have been tested longer ago for heavy metals because it would have saved me so much grief. I have bought detox suppositories to get rid of the heavy metals. It is safer than IV chelation therapy because they do not harm the kidneys and other organs as they leave. It can take any where from 6 months and longer depending on how bad one's problem is. I have been taking suppositories for over 2 and a half months and have noticed a difference but I feel like I still have a long way to go. I have no choice because it is the only way I will get better and live a healthier life. Oh, and get rid of those pesky dark eye circles. By the way, lead toxicity causes anemia and that could be another reason for the dark circles besides the fact that all my organs are having to work harder. My anti-oxidant levels were also extremely low because heavy metals cause free radical damage that can lead to premature aging. Hope all this information was helpful. My advice is to get a 6 hour urine test for heavy metals and a stool test for any bacterial or parasitic problems.